90.1 and Designing High Performance Commercial Kitchen Ventilation Systems BY DON FISHER, P.ENG., RICH SWIERCZYNA,

Transcription

1 90.1 and Designing High Performance Commercial Kitchen Ventilation Systems BY DON FISHER, P.ENG., RICH SWIERCZYNA, The energy intensity associated with operating commercial kitchen ventilation (CKV) systems is well recognized within the HVAC design community and food service industry. This operating cost burden has stimulated energy efficiency design concepts in recent years, including the application of demand-controlled kitchen ventilation (DCKV). 1 Over the past two and a half decades, ASHRAE Technical Committee 5.10, Kitchen Ventilation, developed and maintained a robust Handbook Chapter 2 while its affiliated Standing Standards Project Committee (SSPC) 154 has promulgated a ASHRAE Standard 154, Ventilation of Commercial Cooking Operations. This standard has been a catalyst and technical foundation for overhauling the kitchen ventilation section in the International Mechanical Code (IMC). 3 Effective 2010, ASHRAE Standard 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings, has embraced specific requirements for commercial kitchen ventilation. Subsequently, California Title 24 incorporated similar requirements commencing in Unfortunately, the industry effort to increase the energy efficiency of CKV systems has not had a corresponding impact on improving thermal comfort within kitchens. In striving to reduce energy consumption, engineers have had a long-standing tendency to minimize (or avoid) tempering makeup air, often compromising thermal comfort in the kitchen. Everyone (from employees to design professionals) has acknowledged that commercial kitchens represent a hot working environment, but documentation of sensible and radiant temperatures in commercial kitchens did not exist within the public domain. In response, ASHRAE funded a research project titled Thermal Comfort in Commercial Kitchens 5 that quantified the relatively extreme thermal conditions in 100 commercial kitchens (Figure 1). Don Fisher, P.Eng., is... Rich Swierczyna is... 54

2 Reprinting of this proof for distribution or posting on web sites is not permitted. Authors may request from the editor an electronic file of the published article to reprint or post on their website once the final version has been published. TECHNICAL FEATURE The goal of this article is to discuss the new standard 90.1 requirements in context with designing energy efficient CKV systems that strive to improve comfort conditions in kitchens. Complementary to this article is a publication by the Department of Energy, which also reviews the requirements in Standard 90.1 for kitchen exhaust. 6 Readers are invited to download PHOTO 1 Need caption. this publication as a complement to our perspective on high-performance CKV systems. And just what does high performance mean with respect to a CKV system? We believe that it means a system where the exhaust hood completely captures and contains cooking effluent, where the associated noise levels and energy costs are minimized, and where thermal comfort in the kitchen is consistent with good engineering practices and workplace standards. The format we have chosen for this article is to copy the appropriate sections in Standard 90.1 (shown in italics), followed by our interpretation and supporting industry information. In some cases, we also have paraphrased rational that was included in the foreword to the public review draft of this addendum as it no longer resides within the Standard Commercial Kitchen Exhaust Systems Replacement air introduced directly into the hood cavity of kitchen exhaust hoods shall not exceed 10% of the hood exhaust airflow rate. This first requirement ends the practice of specifying short-circuit hoods, where large percentages (up to 80%) of the replacement (makeup) air is introduced directly into the hood reservoir. Research conducted by the authors 7 confirmed the hypothesis that air introduced directly into the hood cavity competes directly with the thermal plume rising from the cooking equipment, thus requiring an exhaust rate that is higher than that needed to capture and contain (C&C) this thermal plume by itself. Laboratory tests under various configurations of appliances and hoods demonstrated that when the volume of short-circuit air exceeded 10% of the exhaust airflow, the hood would fail to capture and contain. From that point on, any increase in the quantity t o ( F) Zones: Cooking Dishwashing Preparation Casual Institutional QSR FIGURE 1 Average of operative temperature for kitchen type and kitchen zones with 95% confidence interval (100 kitchens). 5 of makeup air introduced into the hood reservoir required a proportional increase in the exhaust rate to maintain C&C. Anecdotally, this has been referred to as the bathtub effect, where the size of the drain [exhaust collar] must to be large enough to accommodate all water pouring into the tub [hood]; otherwise the tub overflows [spills]. While the 10% value from this research (stated earlier) coincides with the 10% maximum stated in , another reason to permit 10% of the replacement air requirement to be supplied directly into the hood cavity is to allow what can be described as high-velocity, low-flow air curtains. This is a design feature of some manufacturers that has been incorporated around the inside perimeter of hoods to enhance capture and containment by essentially extending the lower edge of the hood. As such hoods may be interpreted as a short-circuit design by the AHJ, the threshold limit of 10% clearly permits the specification of these hoods. NOVEMBER 2014 ashrae.org ASHRAE JOURNAL 55

3 Conditioned supply air delivered to any space with a kitchen hood shall not exceed the greater of: a) the supply flow required to meet the space heating or cooling load b) the hood exhaust flow minus the available transfer air from adjacent spaces. Available transfer air is that portion of outdoor ventilation air not required to satisfy other exhaust needs, such as restrooms, and not required to maintain pressurization of adjacent spaces. The rationale behind this requirement is to capitalize on available transfer air (that originated as outside air and has already been heated or cooled) as a contribution to the replacement air requirement of the exhaust hood, potentially reducing the size and energy consumption of the dedicated makeup air unit. In some cases, transfer ducts and/or fans may be incorporated within the design to meet this requirement. In other cases a dedicated MUA unit may be a more cost-effective option and the designer may need to argue that the transfer air was not available from a practical perspective. This is particularly true when the occupancy schedule of the adjacent space is not in sync with operation of the exhaust hood. Section applies to the design of all CKV systems, regardless of airflow capacity, while the following requirements apply only to CKV system design where the exhaust rate exceeds 5,000 cfm (2360 L/s) If a kitchen/dining facility has a total kitchen hood exhaust airflow rate greater than 5,000 cfm then each hood shall have an exhaust rate that complies with Table If a single hood is installed over appliances with different duty ratings, then the maximum allowable flow rate in each section of the hood shall not exceed the Table values for the appliance duty ratings in that section of the hood. Refer to ASHRAE Standard for definitions of hood type, appliance duty, and net exhaust flow rate. The 5,000 cfm (2360 L/s) threshold was established to exempt smaller restaurants where the cost-effectiveness of the energy efficiency requirements could be challenged. The facility has a total kitchen hood exhaust airflow rate greater than 5,000 cfm statement replaced Individual kitchen exhaust hoods larger than 5,000 cfm. This prevented the addition of multiple hoods or hood sections individually operating beneath 5,000 cfm (2360 L/s) to avoid the energy saving methods and specifications required in and for larger capacity systems. TABLE : Maximum net exhaust flow rate, cfm per linear foot of hood length. (From Standard ) TYPE OF HOOD Wall-Mounted Canopy LIGHT DUTY MEDIUM DUTY HEAVY DUTY EXTRA HEAVY DUCT Single Island Double Island (Per Side) Eyebrow Not Allowed Not Allowed Backshelf Pass-Over Not Allowed Exception: a) At least 75% of all the replacement air is transfer air that would otherwise be exhausted. The exhaust airflow rates in Table are 30% below the minimum airflow rates in ASHRAE Standard Ventilation for Commercial Cooking Operations, for unlisted hoods. While the IMC still recognizes unlisted hoods, this airflow limitation in Standard 90.1 effectively outlaws unlisted hoods (for systems greater than 5,000 cfm [2360 L/s]). ASHRAE research has shown that hoods listed per UL Standard and tested per ASTM/ANSI F have exhaust rates that are at least 30% less than those of the unlisted hoods. The following discussion delves into this aspect, presenting third-party performance data for a number of hood designs. Certification bodies (CBs) list and label exhaust hoods that operate with commercial cooking appliances and a fire extinguishing system according to Standard UL 710 Exhaust Hoods for Commercial Cooking Equipment. 8 The Standard includes a construction section and performance section. The construction requirements ensure the hood s integrity during a fire and fan failure. The performance section ensures the hood s ability to capture and contain the cooking effluent and products of combustion under actual cooking conditions. However, in some cases, the cooking challenge may not be as challenging as some real-world conditions. As a result, the ASTM Kitchen Ventilation subcommittee of F26 Food Service Equipment took on a project to develop a test method 9 using enhanced flow visualization techniques to determine the capture and containment exhaust rate for different appliance lines and exhaust hood configurations. This test method was used as the foundation of past CKV research projects 7,10,11,12 as well as a basis for ongoing evaluation of high-performance hoods. 13 Figure 2 presents the results of third-party testing for eight different designs of 10-foot, listed wall-canopy 56

4 Reprinting of this proof for distribution or posting on web sites is not permitted. Authors may request from the editor an electronic file of the published article to reprint or post on their website once the final version has been published. TECHNICAL FEATURE hoods in comparison with a generic unlisted hood operating over a heavy-duty appliance line (two-vat fryer, under-fired boiler and convection oven) in accordance with ASTM In the first set of data the hoods are without side panels while in the second set the hoods have partial side panels. The generic unlisted hood did not include any of the performance enhancing features that the high-performance hoods incorporated (e.g., flow-enhancing flanges). Figure 2 shows that the capture and containment exhaust airflow rate for all eight of the high-performance hoods are at or below the maximum limit of 280 cfm per linear foot of hood (2,800 cfm total) set in Standard The variations in the C&C exhaust rates are due to the differences in individual hood designs. Note that the generic hood exceeded the maximum and that Manufacturer 8 s hood was at the limit for the condition without side panels (due to convective spill from the side of the hood). However, in both cases, the addition of side panels reduced the exhaust rate below the threshold. The bottom line is that many listed hoods will meet the Standard 90.1 exhaust rate requirements for wall-canopy hoods based on both UL 710 listings and ASTM F1704 performance testing. Note, however, that the UL 710 listing cfm numbers must be met in cases where the ASMT F1704 cfm values are lower. The same may not be said for the specification of single-island hoods operating over heavy-duty appliances, which are typically being used for exhibition cooking stations in modern food service facilities. The strong effluent plume that is generated by the heavy-duty appliances is very susceptible to cross drafts, reduced overhang and high-velocity makeup air introduction. As a result, the exhaust airflow rates required to capture and contain such thermal plumes increased significantly. ASHRAE RP-1480, Island Hood Energy Consumption and Energy Consumption Strategies, 12 explored the C&C exhaust airflow rates for single-island canopy hoods. One style of island canopy hood (10 ft by 4 ft [3 m by 1.2 m]) had the filter bank along the rear of the hood while the second style of hood (10 ft by 6 ft [3 m by 1.8 m]) had a V-bank down the center of the hood. The replacement air was supplied through 14 floor-mounted displacement diffusers to emulate transfer air from adjoining spaces and provide a low-velocity air supply, which has been proven to minimize the influence of replacement air on hood performance. The results are shown in Figure 3. Capture and Containment Exhaust Airflow Rate (cfm) 5,000 4,000 3,000 2,000 1,000 0 Combination-Duty Line-Broiler ASHRAE Standard cfm/ft Generic Hood Max Overhang Manufacturer 1 Manufacturer 2 Manufacturer 3 Manufacturer 4 Manufacturer 5 Manufacturer 6 Manufacturer 7 Manufacturer 8 Combination-Duty Line w/ Side Panels-Broiler FIGURE 2 Published manufacturers ASTM test data for the C&C exhaust airflow rates for a 10 ft (3 m) wall-mounted canopy hood over a heavy-duty appliance line. The difficulty in capture and containment is evident by the high exhaust airflow rates. While side panels mitigate some of the cross drafts and improve C&C performance, the data show that it is very difficult for singleisland canopy hoods to stay below the 420 cfm/ft (650 L/s m) limit imposed by Standard However, it is important to note that the data in Figure 3 is for generic, unlisted hoods. Unfortunately, not many manufacturers have used ASTM F1704 to report airflow rates for highperformance island hoods. Our message to the design engineer: Proceed with caution! If a kitchen/dining facility has a total kitchen hood exhaust airflow rate greater than 5,000 cfm then it shall have one of the following: a) At least 50% of all replacement air is transfer air that would otherwise be exhausted. b) Demand ventilation system(s) on at least 75% of the exhaust air. Such systems shall be capable of at least 50% reduction in exhaust and replacement air system airflow rates, including controls necessary to modulate airflow in response to appliance operation and to maintain full capture and containment of smoke, effluent and combustion products during cooking and idle. c) Listed energy recovery devices with a sensible heat recovery effectiveness of not less than 40% on at least 50% of the total exhaust airflow. NOVEMBER 2014 ashrae.org ASHRAE JOURNAL 57

5 Maximizing Transfer Air The replacement air required for commercial kitchen ventilation systems is always 100% of the exhaust air; what goes out must come in! A common design practice has been to supply at least 80% of replacement air using an independent makeup air unit (MAU) with the remaining 20% supplied by conditioned outdoor air from HVAC rooftop units (RTU) serving the kitchen and/ or by transfer air from adjacent spaces. This keeps the kitchen under a negative pressure (relative to the dining room) helping to prevent cooking odors from migrating into the dining area. However, in some climates the replacement air from an independent makeup air unit is not conditioned, which may create uncomfortable conditions (too cold and/or too hot) in the kitchen. In other climates, the makeup air is heated, which in many cases results in simultaneous heating (by the MAU) and cooling (by the RTU) of the kitchen during the shoulder seasons. Conventional design practice does not take full advantage of the relatively high rate of occupancy ventilation air per ASHRAE Standard , Ventilation for Acceptable Indoor Air Quality, that is introduced into the dining room or other areas of the building adjacent to the kitchen. The first option under Section encourages using code-required outdoor air supplied to the dining room as replacement air, thus reducing (or eliminating) the fraction of replacement air from an independent makeup air unit. If this transfer air from the dining room (that otherwise would be exhausted or relieved by rooftop equipment) is 50% (or more) than the exhaust, the CKV design is in compliance with the Section requirement. Obviously, care should be taken to not introduce transfer air at high velocities within the kitchen, potentially cooling food that is staged near transfer windows. If less than 50% is available, it may still be used for the associated benefit, just not as a compliance option under Section And since occupancy ventilation air is conditioned in most cases, transferring it to the kitchen can improve comfort conditions. Several national restaurant chains have adopted this strategy and derive all replacement air for the CKV system from the HVAC system, realizing a positive benefit on thermal comfort in the kitchen. (Coincidently, none of these chains were included in the ASHRAE thermal comfort in commercial kitchens study.) In a case study by the authors (Figure 4), the design exhaust rate was Combination-Broiler Fryer, Broiler, Oven (F,B,O) w/side Panels Combination-Broiler (F,B,O) Heavy-Duty (3 Broilers) Heavy-Duty (3 Broilers) w/side Panels Heavy-Duty (3 Broilers) Minimum Overhang Single Island/Center V-Bank Single Island/Rear Filter ASHRAE Standard cfm/ft 0 2,000 4,000 6,000 8,000 10,000 Exhaust Airflow Rate (cfm) FIGURE 3 C&C Exhaust Airflow Rates for Single Island Canopy Hoods from ASHRAE RP-1480 Island Hood Energy Consumption and Energy Consumption Strategies. successfully reduced from 8,000 cfm to 6,000 cfm (3776 L/s to 2832 L/s) by incorporating best practices (54 in. [1.4 m] deep listed hood, increased overhang, pushing equipment tight to the wall and adding side panels 14 thus meeting the Standard 90.1 requirement with the 3,000 cfm (50%) transfer air Demand Controlled Kitchen Ventilation The second option in Section of the standard recognizes that demand-controlled kitchen ventilation (DCKV) controls are an important consideration for an energy efficient kitchen ventilation system. There are now more than a dozen manufacturers of DCKV systems supporting the authors hypothesis that DCKV is poised to move from best practice to standard practice within the design of commercial kitchens. 1 It is important to recognize that not all CKV systems can be operated at 50% of design flow when appliances are in an idle or ready-to-cook mode (regardless of the control system installed). This is due to the fact that some cooking appliances, such as under-fired charbroilers, exhibit a thermal plume under ready-to-cook conditions that is almost as aggressive as the plume generated by the cooking activity. But there is also the condition (of hood operation) when several appliances have been turned off (including the charbroiler) yet operation of the hood must be maintained (for one appliance remaining on or during the cool-down and 58

6 Reprinting of this proof for distribution or posting on web sites is not permitted. Authors may request from the editor an electronic file of the published article to reprint or post on their website once the final version has been published. TECHNICAL FEATURE cleanup periods). In such cases, it may be feasible to reduce the exhaust rate to the 50% level without repercussion. The question for the designer is which DCKV systems on the market can be responsive to this operating scenario and be in compliance with the Standard 90.1 requirement Energy Recovery The third option in Section recognizes listed air-to-air energy recovery equipment (also referred to a heat recovery ventilation or HRVs) as a compliance path for Thermodynamically, exhaust air heat recovery from kitchen ventilation is very attractive. In practice, however, this waste heat stream is challenged by the fact that the exhaust air is grease laden and subject to NFPA 96 jurisdiction, potentially requiring fire protection and/or wash down systems. Consequently, these systems are more expensive to install ($/cfm) than other HRV applications, challenging the ROI equation. There also is a conflict with Section of the IMC that does not permit HRVs in kitchen exhaust. However, there is now an exception stating that ERV equipment is not limited by this code section if such equipment recovers only sensible heat and utilizes only coil-type heat exchangers. This would allow loop or runaround coil type energy recovery systems for kitchen exhaust. One positive attribute of this HRV application is the typical unbalanced flow condition (i.e., more exhaust than supply air). This inherently increases the thermal effectiveness of the HRV. If applied in combination with a DCKV system, the effectiveness further increases under part-load (reduced airflow) conditions. The elevated exhaust air temperatures combined with the unbalanced flow condition makes heat exchanger frosting unlikely. (Heat exchanger frosting or freezing is an issue in cold climates and a defrost strategy often required.) The unbalanced flow conditions also make meeting the 40% effectiveness threshold very attainable with a runaround-coil style heat recovery system. The DOE article 6 states that DCKV will be a more economically attractive option than heat recovery for 90.1 compliance. We agree. However, the combination of a DCKV system with an HRV may completely eliminate the need for makeup air heating a definite attribute for a LEED building design and the future of CKV energy efficiency. We hope that further research and case studies RTU 3,000 cfm FIGURE 4 Illustration of transfer air used for Standard 90.1 compliance. will build a much needed knowledge base for the HRV potential in CKV design Performance Testing: An approved field test method shall be used to evaluate design air flow rates and demonstrate proper capture and containment performance of installed commercial kitchen exhaust systems. Where demand ventilation systems are utilized to meet , additional performance testing shall be required to demonstrate proper capture and containment at minimum airflow. This section is fundamental to the kitchen exhaust system commissioning and performance verification that protects public health and safety. Hood systems are a field assembly of various components including hoods, fans, replacement air systems, duct and distribution systems and require testing, once installed, to ensure specified system performance is met. This section requires verification of hood system performance and operation, and supports Standard 90.1 purpose and scope. The evaluation of design airflow rates can be determined by applying ASTM F2975 Standard Test Method for Measuring the Field Performance of Commercial Kitchen Ventilation Systems. 15 Protocols have been developed within the standard to measure velocities and apply correction factors to determine exhaust and makeup airflow rates. An approved field test method used to demonstrate capture and containment performance can be found in ASHRAE Standard Ventilation for Commercial Cooking Operations, Section 4.8.2, Type I Hood Capture and Containment Test. In that, it describes a field test where all the appliances under the hood are at operating temperatures, all sources of outdoor air providing makeup air for the hood are operating, and all sources for the MAU 3,000 cfm 6,000 cfm NOVEMBER 2014 ashrae.org ASHRAE JOURNAL 59

7 recirculated air providing conditioning for the space are operating. Capture and containment shall be verified visually by observing smoke or steam produced by actual cooking operation or by simulating cooking by using devices such as smoke candles or smoke puffers. Smoke bombs shall not be used. For a constant-volume system, this test is typically performed during heavy-load conditions. For a DCKV system, the system must be evaluated during heavy, medium and light-load cooking conditions. The worst-case scenario would be a sequence that progresses from no-load or light-load directly to heavy-load to evaluate the system response and its ability to capture and contain the cooking effluent by sufficiently ramping up the airflow rate. Summary The new requirements in Standard 90.1 limit the design exhaust rates for the different styles of hoods and appliance duties. It then requires one of three prescriptive paths, including 50% transfer air, DCKV on 75% of the exhaust or heat recovery on 50% of the exhaust. Facility, operation and economic constraints will dictate the path chosen. But in a perfect world (or maybe for a LEED project), why not incorporate all options. If the heat recovery from the exhaust was used to preheat domestic water in the summer, offsetting system fan energy, future CKV designs could approach net zero energy! Wow, from the most energy intensive system in a restaurant to net zero? We like to dream! But regardless of the limits to energy efficiency, ASHRAE through its technical committees and industry support has moved the North American CKV design world from the dark ages to the front-of-thepack. Furthermore, the inclusion of commercial kitchen ventilation within Standard 90.1 has caught the attention of the HVAC design community. We believe that the CKV system design has taken a positive step forward as a result of the Standard 90.1 initiative. Provided the designer understands the implications, limits and subtleties of the best practice options, one can cost-effectively increase the energy efficiency of CKV systems in concert with improving thermal comfort in the kitchen. References 1. Fisher, D., R. Swierczyna, A. Karas Future of demandcontrolled ventilation for commercial kitchens. ASHRAE Journal 55 (2) ASHRAE Handbook Fundamentals, Chap ICC International Mechanical Code. International Code Council, Washington, D.C. 4. California Energy Commission Building Energy Efficiency Standards, Title Stoops, J., et al Thermal comfort in commercial kitchens. ASHRAE Research Project Final Report. 6. DOE. ANSI/ASHRAE/IES Standard Kitchen Exhaust Brohard, G., D.R. Fisher PE, V.A. Smith PE, R.T. Swierczyna, P.A. Sobiski Makeup Air Effects on Kitchen Exhaust Hood Performance. 8. UL Standard for exhaust hoods for commercial cooking equipment. 6 th ed. Standard ASTM Standard F , Test Method For The Performance Of Commercial Kitchen Ventilation Systems. American Society for Testing and Materials. 10. Swierczyna, R., P. Sobiski, D. Fisher Revised Heat gain and capture and containment exhaust rates from typical commercial cooking appliances (RP-1362). Final Report. 11. Swierczyna, R., D. Fisher, P. Sobiski, T. Cole, M. Bramfitt Effect of appliance diversity and position on commercial hood performance (RP-1202). Final Report. 12. Swierczyna, R., P. Sobiski, D. Fisher, Island hood energy consumption and energy consumption strategies (RP-1480). Final Report. 13. FSTC Kitchen Hood Performance Reports. Fishnick.com. May FSTC CKV Design Guides. Fishnick.com. com/ventilation/designguides/ 15. ASTM Standard test method for measuring the field performance of commercial kitchen ventilation systems. Standard F American Society for Testing and Materials. 60